Lack of stable initiation factor 3 (IF‐3) binding to dimers of the 30 S ribosomal subunits

It is known that initiation factor 3 (IF-3) is endowed with ribosome dissociation factor (DF) activity Recent studies have shown that IF-3 acts as antiassociation factor since the equilibrium between 70 S ribosomes and 50 S and 30 S ribosomal subunits is shifted in favour of the subunits by the binding of IF-3 to free 30 S particles The finding that IF-3 does not stably bind to the 70 S monomers is consistent with this scheme The 30 S-IF-3 complex is dissociated during the physiological formation of the 70 S initiation complex upon addition of 50 S ribosomal subunits in the presence of fMet-tRNA, or during the formation of 70 S couples when 30 S and 50 S particles are 'forced together' at high (15 mM) Mg 2+ concentration Therefore, an antagonism between IF-3 and 50 S particles for the binding to the 30 S ribosomal subunits seems to exist.